Paper webs are commonly dried using a drying apparatus, such as a through-dryer. For example, through-dryers typically operate by contacting heated air with a paper web while the web is supported by a wire or other papermaking fabric. The heated air dries the web as it is transported around a rotating drum. However, one problem associated with conventional methods of drying paper webs with such dryers is that, due to the wetness of the web, the dryers are relatively inefficient and have a low production capacity. The webs are also susceptible to heat-related degradation, which can create various malodorous compounds.
As such, a need currently exists for an improved method of drying a paper web. In particular, a need currently exists for an improved method of drying a paper web that allows the dryer to have an increased production capacity without having a substantially adverse affect on product quality.
The present invention is directed to a method of drying a paper web. In particular, the present invention is directed to a method of providing temperature control of a paper web as it traverses through a through-dryer. For instance, in one embodiment, a paper web is first dried within a first dryer section at an elevated temperature and subsequently dried within a second dryer section at a reduced temperature.
The method of the present invention can generally be utilized with various dryers used in drying paper webs. For instance, a through-dryer can be provided with two dryer sections in accordance with the present invention. A xe2x80x9crelatively wetxe2x80x9d paper web is initially provided to a first dryer section to be dried. As used herein, the phrase xe2x80x9crelatively wetxe2x80x9d generally refers to paper webs having a low solids consistency. For instance, a web may be supplied to the first dryer section at a consistency of less than about 60% (% solids consistency), particularly between about 15% to about 45%, and more particularly between about 20% to about 40%. As the web is moved through the first dryer section, it is partially dried.
From the first dryer section, the web then enters a second dryer section for further drying. In general, the web entering the second dryer section is xe2x80x9crelatively dryxe2x80x9d. As used herein, the phrase xe2x80x9crelatively dryxe2x80x9d generally refers to paper webs having a higher solids consistency than a xe2x80x9crelatively wetxe2x80x9d web. For example, xe2x80x9crelatively wetxe2x80x9d webs having consistencies within the above-mentioned ranges can be dried to consistencies of greater than about 25% (% solids consistency), particularly greater than about 35%, and more particularly between about 45% to about 70%, within the first dryer section to result in a xe2x80x9crelatively dryxe2x80x9d web. Although the exemplary ranges mentioned above for xe2x80x9crelatively dryxe2x80x9d webs and xe2x80x9crelatively wetxe2x80x9d webs are overlapping, such webs should generally be interpreted to have different consistencies. For instance, in some instances, a xe2x80x9crelatively wetxe2x80x9d web may have a consistency of about 35%. In such cases, a xe2x80x9crelatively dryxe2x80x9d web would accordingly have a consistency of greater than about 35%. It should also be understood that, at any given point of a continuous through-drying process, the solids consistency of a web passing therethrough is generally greater than the solids consistency of the web at any previous point of the process.
In accordance with the present invention, the temperatures within the first dryer section and the second dryer section can be selectively controlled to improve the overall capacity of the drying operation. In one embodiment, for example, an elevated temperature can be provided to the first dryer section when the web is relatively wet and a reduced temperature, in comparison to the elevated temperature, can be provided to the second dryer section when the web is relatively dry. For instance, in one embodiment, a temperature between about 400xc2x0 F. to about 500xc2x0 F., and particularly between about 450xc2x0 F. to about 500xc2x0 F., is provided to the first dryer section, while a temperature between about 300xc2x0 F. to about 400xc2x0 F., and particularly between about 300xc2x0 F. to about 350xc2x0 F., is provided to the second dryer section.
Generally, the provision of an elevated temperature to the first dryer section does not cause the temperature of the web to be increased significantly above its xe2x80x9cthermal degradation temperaturexe2x80x9d. As used herein, the xe2x80x9cthermal degradation temperaturexe2x80x9d generally refers to the temperature at which a component (e.g., fiber, lignin, additives, etc.) of a paper web begins to chemically degrade and generate malodorous compounds, as is well known in the art. In particular, when the web is relatively wet, the heated air does not easily pass between the fibers within the web. Instead, most of the heated air flows parallel to the surface of the web and raises the temperature of the web until it reaches the saturation temperature of air for a given humidity, temperature, and pressure. Once the saturation temperature is attained, the heated air then begins to significantly evaporate moisture contained within the web. Accordingly, because the temperature of the relatively wet web is not significantly increased above the saturation temperature of the air when dried at an elevated temperature, the temperature of the web within the first through-dryer section can usually remain less than the xe2x80x9cthermal degradation temperaturexe2x80x9d of the web.
Heat can be supplied to the first dryer section and the second dryer section using a variety of methods and/or techniques. For instance, in some embodiments, a first air channel can supply air at an elevated temperature to the first dryer section, and a second air channel can supply air at a reduced temperature to the second dryer section. The temperature within each air channel may be controlled using a variety of techniques, such as, but not limited to, burners, valves, cooling units, other streams of air, and the like.
Moreover, in some embodiments, a single air channel can supply air to each dryer section. When utilizing a single air channel, the air is typically heated to a certain temperature and then distributed to the dryer sections. For instance, in one embodiment, the air within a single air supply channel is heated to an elevated temperature and distributed to the first dryer section. However, when distributing the heated air to the second dryer section, the temperature of the heated supply air can be lowered to a reduced temperature using a variety of control techniques, such as, but not limited to, a stream of supplemental or recycled air, a cooling unit, etc. Moreover, in some instances, such as when utilizing a stream of air to cool the heated supply air, the reduced temperature can actually vary at different points within the second dryer section. For example, a stream of cool air can be combined with the heated supply air within the second dryer section such that the temperature of the web gradually decreases as it moves through the second dryer section.
In another embodiment, the air within a single air supply channel is heated to a reduced temperature and distributed to the second dryer section. However, when distributing the air to the first dryer section, the temperature of the air can be increased to an elevated temperature using a variety of control techniques, such as, but not limited to, supplemental heated air or a burner. For example, when utilizing a burner, the elevated temperature can be relatively constant. Moreover, in some instances, such as when utilizing a stream of air to heat the supply air, the elevated temperature can actually vary at different points within the first dryer section. For example, a stream of heated air can be combined with the supply air within the first dryer section such that the temperature of the web gradually decreases as it moves through the first dryer section.
Other features and aspects of the present invention are discussed in greater detail below.